He argued that the Universal Declaration on Human Rights specifically defined marriage as a relationship between men and women and accused the coalition of being "staggering arrogant" in seeking to redefine it.

"Since all the legal rights of marriage are already available to homosexual couples, it is clear that this proposal is not about rights, but rather is an attempt to redefine marriage for the whole of society at the behest of a small minority of activists," the head of the Catholic Church in Britain wrote.

He added: "Same-sex marriage would eliminate entirely in law the basic idea of a mother and a father for every child. It would create a society which deliberately chooses to deprive a child of either a mother or a father."

Both Conservative and Liberal Democrat figures jumped to defend their proposals, which will be the subject of a consultation to be launched later this month, in response to the cardinal's comments.

"When civil partnership leg went through, the churches were telling us that was going to be undermining marriage," Tory MP Margot James told BBC1's The Andrew Marr Show.

"That hasn't happened at all - marriages have shown an increase in the last year. I think this is just scaremongering."

Scottish secretary Michael Moore, a Liberal Democrat, insisted: "We're not seeking to change religious marriage and we're not seeking to impose it on religious groups.

"What we're saying is when a couple love each other they should be able to have a civil marriage irrespective of their sexual orientation."

But Cardinal O'Brien said the implications for society would be "immense" and that it was "staggering arrogant" of the government to suggest churches could choose to opt out.

"No government has the moral authority to dismantle the universally understood meaning of marriage," he wrote.

"Imagine for a moment that the government had decided to legalise slavery but assured us that 'no one will be forced to keep a slave'.

"Would such worthless assurances calm our fury? Would they justify dismantling a fundamental human right? Or would they simply amount to weasel words masking a great wrong?"

Labour deputy leader Harriet Harman dismissed his argument that marriage had been exclusively between a man and a woman for hundreds of years.

"We've had prejudice, discrimination and homophobia for hundreds and hundreds of years - it doesn't make it right," she told BBC1's The Andrew Marr Show.

"I don't want anybody to feel this is a licence for whipping up prejudice... I hope it won't have the effect of fuelling or legitimising prejudice or discrimination."